1. Field of the Invention
The present invention relates to an oscillometric-type blood-pressure measuring apparatus which employs, for improving the accuracy of blood-pressure measurements, a pulse-wave detecting device for detecting a pulse wave propagated to a downstream-side portion of an inflatable cuff which is wound around a body portion of a living subject, and which determines a blood pressure of the subject based on the pulse wave detected by the pulse-wave detecting device. The downstream-side portion of the cuff means the portion thereof other than the upstream-side portion thereof.
2. Related Art Statement
Generally, an oscillometric-type blood-pressure (BP) measuring apparatus includes (a) an inflatable cuff which is adapted to be wound around a body portion of a living subject and includes an inflatable bag; (b) a pressure changing device which increases a pressing pressure of the inflatable bag up to a prescribed pressure value which can completely stop the flow of blood through an artery under the cuff, and subsequently slowly decreases the pressure in the bag at a prescribed rate; a pressure sensor which continuously detects the pressure in the bag during the slow deflation of the bag; a pulse-wave filter which extracts a pulse wave from the pressure in the bag, continuously detected by the pressure sensor; and a means for determining, as a systolic blood pressure of the subject, a pressure in the bag at a rising point where respective amplitudes of successive heartbeat-synchronous pulses of the extracted pulse wave significantly greatly changes to increase.
However, the above BP measuring apparatus has the problem that since the rising point of the amplitudes of the pulse wave is indefinite, the determined systolic BP value of the subject may be inaccurate. The reason is that even if the pressing pressure of the cuff is higher than the systolic BP value of the subject, the pulsation of the artery under the cuff may start under an upstream-side portion of the cuff. In particular, in the case where the body portion around which the cuff is wound is difficult to completely stop the flow of blood in the artery thereof, the pulsation of the artery is likely to be large even if the pressing pressure of the cuff may be higher than the systolic BP value of the subject. Therefore, the rising point of the amplitudes of the pulse wave is likely to be indefinite.
To solve the above-indicated problem, it has been proposed to provide a pulse-wave detecting device on an inner side and a downstream side of an inflatable bag of an inflatable cuff and determine a BP value of a living subject based on respective amplitudes of heartbeat-synchronous pulses of a pulse wave detected by the pulse-wave detecting device. For example, the pulse-wave detecting device may be one which includes another or second inflatable bag which is independent of the first inflatable bag for pressing the artery, and is provided on the downstream side of the first bag; and a pressure sensor which detects a pressure in the second bag. A BP value of the subject is determined based on respective amplitudes of heartbeat-synchronous pulses of a pulse wave which is produced in the second bag and is detected by the pressure sensor. Even if the pulsation of the artery may occur under the upstream-side portion of the cuff when the pressing pressure of the cuff is higher than the systolic BP value of the subject, the pulsation does not directly propagate to the second inflatable bag provided in the downstream-side portion of the cuff. Therefore, a definite rising point of the amplitudes of the pulse wave can be found and accordingly an accurate systolic BP value of the subject can be determined.
However, in some cases, the rising point of the amplitudes of the pulse wave detected by the above-described pulse-wave detecting device may be indefinite. In particular, in the case where a pulse wave is detected from an ankle of a living subject, the rising point is likely to be indefinite. The reason is that although the pulse wave which occurs under the upstream-side portion of the cuff when the pressure of the cuff is higher than the systolic BP value of the subject does not directly propagate to the second inflatable bag of the pulse-wave detecting device, the pulse wave is detected by the pulse-wave detecting device via the first inflatable bag. More specifically described, the pulse wave which occurs under the upstream-side portion of the cuff when the cuff pressure is higher than the systolic BP value, causes a pressure oscillation in the first inflatable bag, which in turn is detected by the pulse-wave detecting device. Therefore, the systolic BP value determined based on the amplitudes of the pulse wave detected by the pulse-wave detecting device may be inaccurate.
It is an object of the present invention to provide a blood-pressure measuring apparatus which can measure an accurate systolic blood pressure of a living subject.
The above object has been achieved by the present invention. According to a first feature of the present invention, there is provided an apparatus for measuring a systolic blood pressure of a living subject, comprising an inflatable cuff which is adapted to be wound around a body portion of the subject and includes a first inflatable bag which is inflatable to press an artery of the body portion; a first amplitude determining means for determining a first amplitude of each of a plurality of heartbeat-synchronous pulses of a first pulse wave which is produced in the first inflatable bag when a pressure in the first bag is changed; a pulse-wave detecting device which detects a second pulse wave which propagates from the artery to a downstream-side portion of the cuff as seen in a blood-flow direction in which blood flows in the artery; a second amplitude determining means for determining a second amplitude of each of a plurality of heartbeat-synchronous pulses of the second pulse wave which is detected by the pulse-wave detecting device when the pressure of the first inflatable bag is changed; a ratio calculating means for calculating a ratio of one of each of the first amplitudes determined by the first amplitude determining means and each of the second amplitudes determined by the second amplitude determining means to the other of said each of the first amplitudes and said each of the second amplitudes; and a blood-pressure determining means for determining the systolic blood pressure of the subject, based on the ratios calculated by the ratio calculating means.
In the present BP measuring apparatus, the first amplitude determining means determines a first amplitude of each of a plurality of heartbeat-synchronous pulses of a first pulse wave which is produced in the first inflatable bag when a pressure in the first bag is changed; the second amplitude determining means determines a second amplitude of each of a plurality of heartbeat-synchronous pulses of a second pulse wave which is detected by the pulse-wave detecting device when the pressure of the first inflatable bag is changed; the ratio calculating means calculates a ratio of one of each of the first amplitudes and each of the second amplitudes to the other of the each of the first amplitudes and the each of the second amplitudes; and the BP determining means determines, based on the ratios, the systolic BP value of the subject. When the pressure of the first inflatable bag is higher than the systolic BP value of the subject, the pulse-wave detecting device including the second inflatable bag provided on the downstream side of the first inflatable bag detects the pulse wave only indirectly via the first bag. Thus, the second amplitudes are small. On the other hand, when the pressure of the first bag is not higher than the systolic BP value of the subject, the pulse-wave detecting device detects the pulse wave not only indirectly via the first bag but also directly from the artery. Thus, the second amplitudes are great. However, even if the pressure of the first bag is higher than the systolic BP value, the pressure pulse wave is directly transmitted from the artery to the first bag. Thus, the first amplitudes do not change so greatly as the second amplitude, when the pressure of the first bag is equal to the systolic BP value. Therefore, the amplitude ratios calculated by the ratio calculating means greatly changes when the pressure of the first bag is equal to the systolic BP value, and the BP determining means can determine, based on the amplitude ratios, an accurate systolic BP value of the subject.
According to a second feature of the present invention, the pulse-wave detecting device includes a second inflatable bag which is provided in the downstream-side portion of the cuff wound around the body portion of the living subject, such that the second inflatable bag is located on a downstream side of a first portion of the first inflatable bag and on an inner side of a second portion of the first bag, the second bag having, in the blood-flow direction, a width smaller than a width of the first bag, and wherein the pulse-wave detecting device detects the second pulse wave which is produced in the second bag. Since the present BP measuring apparatus enjoys a simple construction, it can be produced with ease and at low cost.
According to a third feature of the present invention, the blood-pressure determining means determines the systolic blood pressure of the subject, based on the ratios which are calculated, by the ratio calculating means, based on a portion of pre-selected ones of the first amplitudes or the second amplitudes that falls within a predetermined pressure range. Since the BP determining means determines the systolic BP value of the subject, based on the ratios calculated based on only a portion of pre-selected ones of the first amplitudes or the second amplitudes that falls within a predetermined pressure range, the BP determining means does not erroneously determine a systolic BP value in a pressure range different from the predetermined pressure range. Thus, the BP determining means can obtain a more accurate systolic BP value of the subject.
According to a fourth feature of the present invention, the blood-pressure measuring apparatus further comprises a smoothing means for smoothing the ratios calculated by the ratio calculating means, and the blood-pressure determining means determines the systolic blood pressure of the subject, based on the ratios smoothed by the smoothing means. Even if the amplitude ratios calculated by the ratio calculating means may temporarily greatly change, the smoothing means smoothes or eliminates the temporary change of the amplitude ratios, and the BP determining means determines the systolic BP vale of the subject based on the smoothed amplitude ratios. Thus, the BP determining means can obtain a more accurate systolic BP value of the subject.
According to a fifth feature of the present invention, the smoothing means smoothes the ratios which are calculated, by the ratio calculating means, based on a portion of pre-selected ones of the first amplitudes or the second amplitudes that falls within a predetermined pressure range, and the blood-pressure determining means determines the systolic blood pressure of the subject, based on the ratios smoothed by the smoothing means. Since the BP determining means determines the systolic BP value of the subject, based on the ratios calculated based on only a portion of pre-selected ones of the first amplitudes or the second amplitudes that falls within a predetermined pressure range, the BP determining means does not erroneously determine a systolic BP value based on a great change of the amplitude ratios in a pressure range different from the predetermined pressure range. In addition, even if the amplitude ratios may temporarily greatly change in a pressure range different from the predetermined pressure range, the smoothing means smoothes or eliminates the temporary change of the amplitude ratios, and the BP determining means does not determine an erroneous systolic BP vale of the subject based on the temporary change of the amplitude ratios. Thus, the BP determining means can obtain a more accurate systolic BP value of the subject.